ZA200709380B - Production of membranes - Google Patents

Description

¢ *+2007709380

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THIS INVENTION relates, broadly, to the production of membranes which are relatively impervious, ie have a relatively low permeability, with regard to liquid water, but which are relatively permeable, ie have a relatively high permeability, with regard to water vapour. More particularly, the invention relates to a polymeric composition for use in making such membranes; to a method of making such membranes; and to such membranes comprising said polymeric composition.

According to one aspect of the invention there is provided a polymeric composition for use in making membranes which are relatively impervious to liquid water, but which are relatively pervious to water vapour, the polymeric composition comprising a mixture or blend of a thermoplastic polymeric plastics base material and a modifying agent which reduces the stickiness or tackiness of the base material, the modifying agent forming 5 — 40% by mass of the blend or mixture, and the base material forming 60-95% by mass of the blend or mixture. it is well-known to persons skilled in the art that barrier membranes of the type in question, suitable for example for use in waterproof clothing or rainwear which breathes with regard to gases and vapours, are characterized by desirably low permeability to liquid water, to which they form a relatively impervious barrier, on the one hand, and by desirably high permeability with regard to water vapour, to which they form a relatively pervious barrier, on the other hand. Generally, the permeability of such membranes with regard to water vapour should be at least 100 times higher than the permeability of such membranes with regard to liquid water.

The base material may be selected from the group of thermoplastic elastomers - consisting of polyurethanes, polyamides, polyesters, blends of two or more thereof,

C:\MSWord\MyDocuments\CSIR-BreathingMembrane\Eil 31/10/2007 copolymers of two or more thereof, and copolymers of one or more thereof with one or more polyethers.

The base material is preferably hydrophilic. Preferred base materials include hydrophilic elastomeric base materials selected from the group consisting of polyurethanes, polyesters, blends of two or more thereof, copolymers of two or more thereof, and copolymers of one or more thereof with one or more polyethers, in particular base materials which comprise both hard segments and soft segments being suitable. The base material will typically form the balance, by mass, of the composition according to the invention, excluding the modifying agent and excluding any additives such as the additives mentioned hereunder, the base material usually forming 60 — 90% by mass, preferably 70 — 80%, of the composition, and the modifying agent preferably forming 20 — 30% thereof.

In turn the modifying agent may be selected from the group of natural and semi- synthetic linear hydrophilic thermoplastic materials consisting of polysaccharides (such as cellulose and its derivatives and starch and it derivatives), porous materials (such as diatomites, hydrotalcites, modified diatomites and hydrotalcites and nanoparticulate clays), and blends or mixtures of any two or more thereof. Naturally, synthetic materials may be used instead, provided that, if they are thermoplastic, they have the appropriate properties mentioned above such as the required hydrophilicity and the required linearity, and provided that, if they are not : thermoplastic, they have the required porosity.

The polymeric composition of the present invention comprising the blend of the base material and the modifying agent is preferably a substantially homogeneous blend/mixture and may include various additives dispersed therein, preferably homogeneously, which may be selected from the group consisting of compatibilizers for assisting in mixing of the base material and the modifying agent during blending of the composition, flame retardants, pigments, bactericides, fungicides, antiviral agents and fillers such as calcium carbonate particles or vermiculite.. These additives will typically be used in the usual proportions known in the art. However, in particular filler particles, when used, may form from 1 — 30% by mass of the

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@ 82007/053 54 polymeric composition; and the filler particles will typically have particle size of the usual value encountered in the art.

According to another aspect of the invention there is provided a method of making the polymeric composition of the present invention, the method comprising the steps of: melting the polymeric base material; and admixing the molten polymeric base material with the modifying agent.

If the modifying agent is thermoplastic the method may include melting the modifying agent before mixing it with the molten base material. The mixing is preferably until substantially homogeneous.

The mixing may involve the preliminary step of mixing together the base material and modifying agent in particulate from, conveniently in peliet from or in powder form, followed by subjecting the base material (and the modifying agent if thermoplastic) to : melting, in turn followed by subjecting the mixture containing the molten base material to shear, for example by extruding the mixture, to render the mixture into a substantially homogeneous mix, the molten material then being allowed to solidify, after which, for convenience, the mixture can be pelletized. Once again, if the modifying agent is thermoplastic, it is preferably melted together with the base material, the homogeneous mix, after it has been subjected to shear, being allowed to solidify.

As a preliminary step, when the modifying agent is starch, it may be rendered in- - thermoplastic from by melt-processing it with water and glycerol, together optionally with the usual optional additives such as stearates or the like, which melt-processing plasticizes the starch while rendering it thermoplastic so that miscibility and blending thereof with the polymeric base material is promoted. So : The invention extends to a membrane which is relatively impervious to liquid water but is relatively pervious to water vapour, the membrane having a thickness in the range 10-90um and comprising a polymeric composition in accordance with the

C:\MSWord\MyDocuments\CSIR-BreathingMembrane\Eil 31/10/2007 present invention as defined above, being a blend or mixture of said polymeric base material and said modifying agent.

The invention extends further to a method of making the above membrane, the method comprising the steps of forming a film from the composition, and subjecting the film to stretching.

While forming the film may in principle be by casting, it is preferably by blowing. E

The invention will now be described, by way of non-limiting illustrative Example, with reference to the following Examples:

EXAMPLE 1 :

An amount of 1kg of Amioca (trade name) starch (100% amylopectin starch), obtained from National Starch and Chemical Company of Bridgewater, New Jersey,

USA, was mixed with 4kg of a copolymer of a polyether and a polyurethane, the copolymer being available from Dow Chemical Company of Midland, Michigan, USA, and being marketed under the trade name “Pellethane 2103-70A". The mixture was extruded in a co-rotating twin screw extruder with a L/D (length:diameter ratio) of 42 using a flat temperature profile of 160°C and a screw speed of 150 rpm followed by pelletizing with a side-cutting pelletizer. The pelletized material was fed into a film blower with a flat temperature profile of 175°C, from which a film with a thickness of about 22um was obtained. Water vapour permeation through the film was measured using the Spanish FNM 817 method employing an aluminium cup with a 35mm - 25 internal height and 65mm internal diameter, the permeation obtained being about 2772 g/m2.day, that of pure Peliethane being about 2353 g/m2.day for a 27um film.

Liquid water permeation remained substantially unchanged.

EXAMPLE 2

An amount of 1kg of flux calcinated diatomite (fossilised diatoms), obtained from

FRM Minerals of 15 Adcock Street, Chamdor, South Africa, was mixed with 4kg of

Pellethane 2103-70A. The mixture was extruded employing the same method as in

Example 1 followed by film blowing to obtain a thickness of about 28um. Water vapour permeation measured in the same way as in Example 1 was found to be

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Eomtrgyem, - about 2130g/m?.day, that of pure Pellethane being about 2353g/m?.day for-a 27um film. Liquid water permeation again remained substantially unchanged.

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Claims (16)

1. CLAIMS NUS

   “1. A polymeric composition for use in making membranes which -are relatively impervious to liquid water, but which are relatively pervious to water vapour, the polymeric composition comprising a mixture or blend of a thermoplastic polymeric plastics base material and a modifying agent which reduces the stickiness or tackiness of the base material, the modifying agent forming 5-40% by mass of the blend or mixture, and the base material forming 60-95% by mass of the blend or mixture. :
2. 2. A composition according to Claim 1, wherein the base material is selected from the group of thermoplastic elastomers consisting of polyurethanes, polyamides, polyesters, blends of two or more thereof, copolymers of two or more thereof, and copolymers of one or more thereof with one or more polyethers.
3. 3. A composition according to Claim 2, wherein the base material is hydrophilic.
4. 4. A composition according to any one of Claims 1 to 3 inclusive, wherein the base material forms 70-80% by mass of the composition, while the modifying : agent forms 20-30% by mass thereof.
5. 5. A composition according to any one of Claims 1 to 4 inclusive, wherein the modifying agent is selected from the group of natural and semi-synthetic linear hydrophilic thermoplastic materials consisting of polysaccharides, porous materials, and blends or mixtures thereof.
6. 6. A composition according to any one of Claims 1 to 4 inclusive, wherein the modifying agent is a synthetic material.
7. 7. A composition according to any one of Claims 1 to 6 inclusive, wherein the mixture or blend is substantially homogeneous, and which includes, dispersed therein, at least one additive selected from the group consisting of a compatibilizer C:\MSWord\MyDocuments\CSIR-BreathingMembrane\Eil 31/10/2007

   * %22007/ 09380 for assisting in mixing of the base material and the modifying agent during blending oo | of the composition, a flame retardant, a pigment, a bactericide, a fungicide, an antiviral agent and a filler.
8. 8. A method of making a polymeric composition for use in making membranes which are relatively impervious to liquid water, but which are relatively pervious to water vapour, the method comprising the steps of: : melting a thermoplastic polymeric plastics base material; and admixing the molten polymeric base material with a modifying agent which reduces the stickiness or tackiness of the base material, the modifying agent forming 5-40% by mass of the blend or mixture, and the base material forming 60-95% by mass of the blend or mixture.
9. 9. A method according to Claim 8, wherein the mixing involves the preliminary step of mixing together the base material and the modifying agent in particulate from, subjecting the mixture of the base material and the modifying agent to melting, subjecting the mixture containing the molten base material to shear, to render the mixture into a substantially homogeneous mix, allowing the molten material to solidify, and thereafter pelletizing the mixture.
10. 10. A membrane which is relatively impervious to liquid water but is relatively pervious to water vapour, the membrane having a thickness in the range 10-90um and comprising a polymeric composition as claimed in any one of Claims 1 to 7 inclusive, being a blend or mixture of the polymeric base material and the modifying agent.
11. 11. A method of making the membrane as claimed in Claim 10, the method comprising the steps of forming a film from the composition, and subjecting the film to stretching.
12. 12. A method according to Claim 11, wherein forming the film is by blowing. C:\MSWord\MyDocuments\CSIR-BreathingMembrane\Eil 31/10/2007

    9 Fomor,
13. 13. A polymeric composition according to any one of Claims 1 to 7, substantially as herein described and illustrated, with reference to the examples.
14. 14. A method of making a polymeric composition according to Claim 8 or Claim 9, substantially as herein described and illustrated, with reference to the examples.
15. 15. A membrane according to Claim 10, substantially as herein described and illustrated, with reference to the examples.
16. 16. A method of making a membrane according to Claim 11 or Claim 12, substantially as herein described and illustrated, with reference to the examples. DATED THIS 31ST DAY OF OCTOBER 2007. Pos APPLICANTS PATENT ORNEYS C:\MSWord\MyDocuments\CSIR-BreathingMembrane\Eil 31/10/2007